Liquid scintillators

ABSTRACT

The counting efficiency of liquid scintillators containing toluene as the solvent for a primary solute or activator and for a secondary solute or spectrum shifter is improved, particularly in the presence of water, for example, when the isotope to be detected is an aqueous solution, by adding to the liquid scintillator a hygroscopic substance which is soluble in toluene in an amount sufficient to absorb and/or emulsify the water that is present. Such hygroscopic additive is preferably cellulosic and may be selected from the group consisting of cellulose, carboxymethyl cellulose, cellulose phosphate, diethylaminoethyl cellulose, benzyl diethylaminoethyl cellulose and epichlorohydrin triethanolamine cellulose. The additive for improving the counting efficiency may advantageously include, apart from the cellulose and/or cellulose derivative, a small amount of sodium or calcium carbonate or bicarbonate. The improvement in counting efficiency resulting from the mentioned addition is particularly large when the scintillators liquid includes alcohol as well as toluene.

United States Patent Shamoo 45] July 11, 1972 [54] LIQUID SCINTILLATORS[22] Filed: June 30, 1971 21 Appl. No.: 158,554

Related US. Application Data [63] Continuation-impart of Ser. No.874,979, Nov. 7,

1969, abandoned.

[52] US. Cl. ..252/30l.2 R, 250/71, 250/7l.5, 250/83, 250/83.l [51] Int.Cl ..G0lt l/20, C09k 1/02 [58] FieldofSearch ..252/301.2 R, 83, 83.1,71,252/715 Blair et al., Anal. Biochem. 3( I962) p. 221- 229. F indeis etal., Molecular Crystals, 1968, Vol. 4, p. 385- 388.

Primary Examiner-Tobias E. Levow Assistant Examiner-A. P. DemersAttorney-Alvin Sinderbrand et al.

[57] ABSTRACT The counting eficiency of liquid scintillators containingtoluene as the solvent for a primary solute or activator and for asecondary solute or spectrum shifter is improved, particularly in thepresence of water, for example, when the isotope to be detected is anaqueous solution, by adding to the liquid scintillator a hygroscopicsubstance which is soluble in toluene in an amount sufiicient to absorband/or emulsify the water that is present. Such hygroscopic additive ispreferably cellulosic and may be selected from the group consisting ofcellulose, carboxymethyl cellulose, cellulose phosphate,diethylaminoethyl cellulose, benzyl diethylaminoethyl cellulose andepichlorohydrin triethanolamine cellulose. The additive for improvingthe counting efliciency may advantageously include, apart from thecellulose and/or cellulose derivative, a small amount of sodium orcalcium carbonate or bicarbonate. The improvement in counting efficiencyresulting from the mentioned addition is particularly large when thescintillators liquid includes alcohol as well as toluene.

12Clai1m,NoDrawings 1 LIQUID SCINTILLATORS This application is acontinuation-in-part of my copending application for US. Letters Patent,Ser. No. 874,979, filed Nov. 7, 1969 now abandoned.

This invention relates generally to liquid scintillators, and isparticularly directed to improvement of the counting efficiency ofliquid scintillators which employ toluene as the solvent for the primaryand secondary solutes thereof.

Liquid scintillators are commercially available for use in the detectionof nuclear radiation, particularly beta radiation, by means of liquidscintillation counters, for example, of the type referred to by thedesignation Beckman LS230. These liquid scintillators comprise asolvent, usually either toluene or dioxane, containing a primary soluteor activator, such as PPO (2,5-diphenyl-oxazole), which collects theenergy absorbed by the solute and emits ultraviolet radiation, and asecondary solute or spectrum shifter, such as, POPOP[ l,4-bis2-(5-phenyloxazolyl)-benzene], which converts the ultraviolet radiationemitted by the activator into. visible light which is detectable by aphotomultiplier tube.

Although toluene based scintillators are generally preferred to dioxanebased scintillators for reasons given below, the toluene basedscintillators have the important disadvantage of experiencing a majorreduction of counting efficiency or quenching when even small amounts ofwater are present in the scintillator, for example, when the isotopewhose radiation is to be detected is present in an aqueous solution.This quenching action increases with decreases in the level of thenuclear energy to be detected, and therefore leads to great difficultiesin detecting low energy nuclear radiation, for example, from H inaqueous solution. The described quenching action occurs when the amountof water introduced into the toluene based scintillator is as small as afew microliters of water in cc of toluene. In the article entitled Useof Filter Paper Mounting for Determination of the Specific Activity ofGluconate -C by Liquid Scintillation Assay" by Alberta Blair and StantonSegal, appearing in Analytic Biochemistry 33, (1962), at pages 221 to229, the problem of liquid scintillation assay of substances, such aspotassium gluconate C, that are insoluble in systems usually employedfor water-soluble compounds was discussed. The article proposed that theisotope-containing gluconate solution be applied to filter paper whiledirecting a jet of warm air against the area of the paper spotted withthe solution, whereby to evaporate the solvent including any watercontained therein and to leave the crystalline gluconate C impregnatedor locked in the filter paper. The thus impregnated filter paper wasthen immersed in the scintillation liquid constituted by PPO and POPOPin toluene and counting effected in a conventional liquid scintillationcounter. At the conclusion of the counting, the filter paper was removedfrom the scintillation liquid and no C activity was detected in thelatter indicating that the gluconate -C remained impregnated in thefilter paper and that there was not any appreciable disolving of thecellulose of the filter paper in the scintillation liquid. Further, theidentified article suggested that, after counting, the filter papercould be repeatedly washed with toluene to remove traces of phosphortherefrom leaving the gluconate to be eluated with water andcrystallized from solution for further degradative procedures. This, itis clear that Blair and Sega] did not seek to avoid the reduction ofcounting efficiency or quenching that is encountered in the use ofscintillation liquid based on toluene when the isotope to be detected ispresent in an aqueous solution that is merely introduced into thescintillation liquid.

Although dioxane based scintillators are not quenched by the presence ofsuch small amounts of water, and hence have been used when the isotopeto be detected and measured is in an aqueous solution, such dioxanebased scintillators have a number of important disadvantages. Amongthese disadvantages are: the production of chemiluminescence whenproteins or the like are present in the aqueous solution containing thesource of nuclear radiation, whereby the accuracy of the measurement ofthe nuclear radiation is disturbed; a substantial quenching action whensubstances other than water are added to the liquid scintillator; andinstability at room or ambient temperatures, so that dioxane basedscintillators might require the use of refrigerated liquid scintillationcounters.

Accordingly, it is an object of this invention to provide liquidscintillators which avoid the above mentioned disadvantages of theexisting toluene and dioxane based scintillators.

Another object is to provide toluene based scintillators which are notsubstantially quenced by the presence of water therein so that suchtoluene based scintillators can be effectively employed for thedetection and measurement of even low energy nuclear radiation fromsources in aqueous solutions.

In accordance with this invention, the quenching of toluene basedscintillators by the presence of water therein is minimized and thecounting efficiency is substantially improved by adding to the toluenebased scintillator a hygroscopic, preferably cellulosic substance whichis soluble in the toluene in an amount sufficient to absorb and/oremulsify at least a substantial part of the water that is present. Theadditive for improving the counting efiiciency of the toluene basedscintillator in the presence of water may consist of one or more of suchcellulosic substances, or of the combination of the latter withsimilarly small amounts of sodium or calcium carbonate or bicarbonate.

The toluene based scintillators having their counting efficienciesimproved by the additives according to this invention, particularly inthe presence of water, may contain PPO as the primary solute, inconcentrations of 4 to 30 gm/liter of toluene and POPOP as the secondarysolute, in concentrations of 0.1 to 1.0 gm/liter of toluene. The amountof the cellulosic substance added to prevent, or at least substantiallyminimize the quenching by the presence of water is preferably at leastsufficient to absorb and/or emulsify all of the water to be introducedinto the liquid scintillator, and the maximum amount of the cellulosicsubstance is preferably no more than the amount that will saturate thetoluene based scintillator therewith. In the case of a toluene basedscintillator containing 4 gm. of PPO and 0.5 gm. of POPOP dissolved in1.0 liter of toluene, 1 gm. of the cellulosic substance per liter oftoluene is sufficient to absorb and/or emulsify l5 microliters of waterper 10 c.c. of the toluene based scintillation liquid. The increase incounting efficiency of the scintillator is not adversely affected by anincrease in the amount of the additive beyond that required forabsorbing and/or emulsifying the water content up to an additiveconcentration of about 10 gm per liter of toluene based scintillator.Substantially beyond the foregoing maximum concentration of theadditive, a progressive quenching action is encountered.

Among the cellulosic substances that have been found to be particularlysuitable for increasing the counting efiiciencies of toluene basedscintillators, particularly in the presence of water, are cellulose,carboxymethyl cellulose (CMC), cellulose phosphate, diethylaminoethylcellulose (DEAE-cellulose), benzyl-diethylaminoethyl cellulose(benzyl-DEAE-cellulose) and epichlorohydrin triethanolamine cellulose(EC- TEOLA-cellulose).

It has been found that the relative improvement in counting efficiencyof toluene based scintillators, particularly in the presence of water,achieved by the additions thereto according to this invention increasesas the energy of the nuclear radiation to be detected decreases. Thus,the relative improvement of counting efficiency with H as the radiationsource will be greater than the relative improvement with C as thesource, and the relative improvement with C as the source will begreater than the relative improvement with l as the source. Moreparticularly, when the radiation source is P in aqueous solution, thecounting efficiency of a toluene based scintillator is increased by20-30% by the inclusion of additives according to this invention,whereas the same scintillator undergoes increases in counting efficiencyof the order of 100-200 percent and 200-900 percent when C and 1'1 arethe radiation sources, respectively. Therefore, toluene basedscintillators with additives according to this invention areparticularly suited for detecting and measuring radiation from lowenergy sources, such as H, in aqueous solutions.

Further, it has been found that the increase in counting efficiency isgreatest when, in addition to the cellulosic additive, the toluene basedliquid scintillator has included therein a substantial quantity of analcohol, for example, from 100 to 400 c.c. of absolute alcohol per literof toluene. This is surprising in view of the disclosure of thequenching properties of a1- cohols in liquid scintillator solutions byA.F. Findeis and J .A. Lubkowitz appearing at pages 385-401 of MolecularCrystals, 1968 Vol. 4, published by Gordon and Breach SciencePublishers.

When toluene based scintillators have additives included therein toincrease the counting efficiency thereof, particularly in the presenceof water, according to this invention, the background counts areunchanged, that is, the background counts are the same with and withoutthe additives. Further, the increased counting efficiency is stable,that is, the counts resulting from the increased efiiciency remainunchanged when testing the same samples with the same scintillators forat least one week after the incorporation of the described additivestherein.

The exact mechanism by which the additives introduced into toluene basedscintillators according to this invention serve to increase the countingefficiency thereof in the presence of water is not fully understood.However, it is believed that such additives, when dissolved in thetoluene, or partly dissolved and partly suspended in the toluene in thecase of an amount of the additive greater than that required to actuatethe toluene, collect or absorb the small amount of aqueous solutioncontaining the radiation source material and thereby promote improvedmixing or emulsifying of the aqueous solution with the toluene. Byreason of such improved mixing, the droplets of water are reduced insize, whereby the beta radiation from the isotope contained in thedroplets of water collected by the dissolved additive travels overrelatively shorter path lengths into the toluene so that the lattersolvent for the activator (PPO) and the specrrum shifter (POPOP) canmore efficiently absorb such radiation.

The invention will now be further described with reference to thefollowing examples which are merely illustrative:

EXAMPLE I A. A toluene based scintillator containing 4 gm. of PPO and0.5 gm. of POPOP dissolved in 1.0 liter of toluene was employed in aBeckman LS230 liquid scintillation counter, and beta radiation sourcesor samples (a), (b), (c), (d), (e) and (f) constituted by progressivelylarger amounts of C were introduced into the toluene based scintillator,in the absence of water, with the counts per minute (CPM) indicated bythe counter being read for each sample.

B. The foregoing tests were repeated for each sample, but with each ofthe samples (a), (b), (c), (d), (e) and (f) being present in microlitersof an aqueous solution which is introduced into 10 c.c. of the toluenebased scintillator.

C. The tests indicated in B above were repeated, but with the aqueoussolutions of the several radiation source samples being introduced intoa toluene based scintillator having the composition indicated above andto which there was added carboxymethyl cellulose (CMC) in aconcentration of about 10 gm. per liter of toluene so as to obtain asaturated solution of the CMC in toluene, with such amount of CMC beingsufficient to substantially absorb the water present.

The results of the above tests IA, 1B and 1C were as follows:

Radi- Percent ation Counts Per Minute (CPM) improvement of Sample Test1A Test 18 Test 1C lC over 1B a 132 55.6 b 1795 1075 1973 83.6 c 30851910 3366 76.3 d 6370 2653 5620 112.0 c 15470 6444 12850 99.6 f 223205580 13020 133.3

From the above, it will be apparent that the presence of water in theknown toluene based scintillator (Test 1B) has a substantial quenchingaction and that such quenching action is substantially minimized and thecounting efficiency is very greatly improved by the addition to thetoluene based scintillator of carboxymethyl cellulose (Test 1C).

EXAMPLE ll Tests 1C above were repeated, but with the carboxymethylcellulose additive being replaced by a substantially equal amount ofcellulose phosphate, and the results were as follows:

Radiation CPM improvement of Sample Test 11 11 over 1B EXAMPLE lll Tests1C above were repeated, but with the carboxymethyl cellulose additivebeing replaced by a substantially equal amount of cellulose, and theresults were as follows:

Test 1C above were repeated, but with the carboxymethyl celluloseadditive being replaced by a substantially equal amount ofdiethylaminoethyl cellulose, and the results were as follows:

Radiation CPM k improvement of Sample Test 1V [V over 18 EXAMPLE V TestsIC above were repeated, but with the carboxymethyl cellulose beingreplaced by a substantially equal amount of benzyl diethylaminoethylcellulose, and the results were as follows:

Radiation CPM k improvement of Sample Test V V over 18 EXAMPLE Vl Tests1C above were repeated, but with the carboxymethyl 5 cellulose beingreplaced by a substantially equal amount of epichlorohydrintriethanolamine cellulose (ECTEOLA), and the results were as follows:

Radiation CPM improvement of Sample Test V1 V1 over 13 a 92 8.2 b 144033.9 c 2805 46 .9 15 d 5830 120. e 14200 120. f 19350 243 EXAMPLE VIITests 1C above were repeated, but with the additive to the toluene basedscintillator also containing an amount of epichlorohydrintriethanolamine cellulose substantially equal to the amount of thecarboxymethyl cellulose additive and the results were as follows:

Radiation CPM improvement Sample Test VII of V11 over 113 a 106 24.7 b1580 46.8 c 3140 64.4 d 6360' 140. e 15450 140. f 21600 287.

EXAMPLE V111 Tests IA, 18 and 1C were repeated as tests VlIlA, VllIB andVlllC, respectively, but using beta radiation sources or sam- 40 ples(g), (h), (i) and (j) constituted by progressively larger amounts of H,and the results .thereof were as follows:

improvement of VlllC Radiation CPM over Sample Test Test Test over VlllAVlllB VlllC VlllB g 1630 272 1390 s10 h 2420 298 2230 750 i 4280 5203760 725 j 7760 796 6960 875 The above results demonstrate that, whenusing low energy beta emitters, such as H, the quenching effect due tothe presence of water in the toluene based scintillator is very large(compare results of W118 with VIIIA), and further that, in the case ofsuch low energy beta emitters, the additives introduced into the toluenebased scintillator, are particularly effective in minimizing suchquenching effect and in achieving a counting efficiency approaching thatrealizable with the toluene based scintillator in the absence of water.

EXAMPLE 1X The test with radiation sample (d) in Example VI wasrepeated, but with the further addition to the toluene base scintillatorof 250 c.c. of absolute alcohol for each liter of toluene, and there wasindicated 8,500 CPM. This is to be compared with 6,370 CPM in the caseof Test [A (no additive and in the absence of water), 2,653 CPM in thecase of Test 18 (no additive in the presence of water) and 5,830 CPM inthe case of Test V1 (epichlorhydrin triethanolamine cellulose as theadditive in a scintillator having only toluene as its solvent). Thus,

by the addition of alcohol to the scintillator the counting efficiencywas improved by about 310 percent over that of the toluene basedscintillator alone in the presence of water.

EXAMPLE X The test with radiation sample (d) in Example V1 was repeated,but with the further addition to the toluene based scintillator ofcalcium carbonate in an amount of about 1 gm. per liter of toluene, andthere was indicated about 8,750 CPM, representing approximately apercent improvement in counting efficiency over that realized with theaddition to the toluene based scintillator of epichlorohydrintriethanolamine cellulose alone. Similar results were obtained with theaddition to the toluene based scintillator of calcium bicarbonate,sodium carbonate or sodium bicarbonate in addition to theepichlorohydrin triethanolamine cellulose.

EXAMPLE XI The tests described in Example 1 were repeated with otherbeta radiation sources or samples (a), (b), (c), (d) and (e) containingprogressively larger amounts of C. As before, such tests involved aseries (A) in which the samples were introduced into the describedtoluene based scintillator in the absence of water, a series (B) inwhich each sample was present in 15 microliters of an aqueous solutionintroduced into 10 c.c. of the toluene based scintillator, and a series(C) in which the aqueous solutions of the several radiation sourcesamples were introduced into the toluene based scintillator to whichthere was added carboxymethyl cellulose (CMC) in a concentration ofabout l gm. per liter of toluene.

The results of the above tests XlA, X18 and XlC were as follows:

The series of tests identified as XlC above were repeated, but with thecarboxymethyl cellulose (CMC) being added to the toluene basedscintillator in a concentration of about 10 gm. per liter of toluene,and the results were as follows:

Radiation CPM improvement of Sample Test X11 X11 over X! Thisdemonstrates that merely increasing the concentration of CMC in thetoluene based scintillator from 1 gm. to 10 gm. per liter of toluenedoes not materially alter the increase in counting efficiency attainedby this invention. However, as demonstrated by the following examples,the advantages of this invention are not realized and, in fact, theadded cellulosic substance has a quenching action of its own thatreduces the counting efficiency below that encountered with the toluenebased scintillator alone in the presence of water, if the amount ofadded cellulosic substance very substantially exceeds the amount thatcan be dissolved in the toluene.

'1 EXAMPLE Xlll The series of tests identified as XlC above wererepeated, but with the CMC being added to the toluene based scintillatorin a concentration of 100 gm. per liter of toluene, and the results wereas follows:

The series of tests identified as XIC above were repeated, but with theCMC being added to the toluene based scintillator in a concentration of200 gm. per liter of toluene, and the results were as follows:

Radiation CPM improvement of Samples Test XIV XlV over XlB It will beapparent that, in accordance with this invention, very substantialimprovements are effected in the counting efficiency of toluene basedscintillators in the presence of water.

In the specific applications of the invention presented in the aboveexamples, the cellulosic additive has been described as being introducedinto the toluene based scintillator prior to the introduction of thebeta emitter in aqueous solution. However, it is contemplated that thecellulosic additives mentioned herein may be provided in sheet form, sothat a sheet of such cellulosic material may be used to wipe a surfacesuspected of being contaminated with a beta emitter after wetting suchsurface with water. Then, the sheet of cellulosic material which hasabsorbed the water and picked up any contaminant from the suspectedsurface is dissolved in the toluene based scintillator and the presenceof any beta emitting contaminant is detected in a liquid scintillationcounter. Of course, the cellulosic material in sheet form must becapable of being readily dissolved in the toluene based scintillator,and the amount of such material should be sufficient to absorb the waterpresent and not greatly in excess of the amount of the cellulosicmaterial that can be dissolved in the toluene based scintillator.

Although illustrative examples of the invention have been described indetail herein, it should be understood that the invention is not limitedto those precise examples and that various changes and modifications canbe effected therein by one skilled in the art without departing from thescope or spirit of the invention.

What is claimed is:

1. in a liquid scintillator having an activator and a spectrum shifterdissolved in toluene and being adapted to detect beta radiation in thepresence of water, a hygroscopic substance added to said liquidscintillator in an amount sufficient to substantially absorb said waterthat is present and being soluble in said toluene at least in saidamount, said hygroscopic substance being selected from the groupconsisting of cellulose, carboxymethyl cellulose, cellulose phosphate,diethylaminoethyl cellulose, benzyl diethylaminoethyl cellulose,epichlorohydrin triethanolamine cellulose, and mixtures thereof.

2. A liquid scintillator according to claim 1, in which said amount ofthe hygroscopic substance is sufficient to provide a saturated solutionthereof in said toluene.

3. A liquid scintillator according to claim 1, in which said amount ofthe hygroscopic substance is in the range between substantially 1.0 and10.0 gm. per liter of said toluene.

4. A liquid scintillator according to claim 1, in which there is furtheradded, in an amount approximately equal to that of said hygroscopicsubstance, a substance selected from the group consisting of calciumcarbonate and bicarbonate and sodium carbonate and bicarbonate.

5. A liquid scintillator according to claim 1, in which alcohol is addedto said scintillator in an amount between and 400 c.c. per liter of saidtoluene.

6. A liquid scintillator according to claim 1, in which said activatoris 2,5-diphenyloxazole and said spectrum shifter is l,4-bis2-(5-phenyloxazolyl)benzene.

7. A method of improving the counting efficiency, in the presence ofwater, of a liquid scintillator having an activator and a spectrumshifter dissolved in toluene, comprising introducing into said liquidscintillator a hygroscopic substance in an amount sufficient tosubstantially absorb said water that is present and being soluble insaid toluene at least in said amount, said hygroscopic substance beingselected from the group consisting of cellulose, carboxymethylcellulose, cellulose phosphate, diethylaminoethyl cellulose, benzyldiethylaminoethyl cellulose, epichlorohydrin triethanolamine cellulose,and mixtures thereof.

8. The method according to claim 7, in which said amount of thehygroscopic substance is sufiicient to provide a saturated solutionthereof in said toluene.

9. The method according to claim 7, in which said amount of thehygroscopic substance is in the range between substantially 1.0 and 10.0gm. per liter of said toluene.

10. The method according to claim 7, in which there is further added, inan amount approximately equal to that of said hygroscopic substance, asubstance selected from the group consisting of calcium carbonate andbicarbonate and sodium carbonate and bicarbonate.

11. The method according to claim 7, further comprising adding to saidscintillator an amount of alcohol that is between 100 and 400 c.c.thereof per liter of said toluene.

12. The method according to claim 7, in which said activator is2,5-diphenyl-oxazole and said spectrum shifter is 1,4-bis2-(5-phenyloxazolyl)-benzene.

2. A liquid scintillator according to claim 1, in which said amount ofthe hygroscopic substance is sufficient to provide a saturated solutionthereof in said toluene.
 3. A liquid scintillator according to claim 1,in which said amount of the hygroscopic substance is in the rangebetween substantially 1.0 and 10.0 gm. per liter of said toluene.
 4. Aliquid scintillator according to claim 1, in which there is furtheradded, in an amount approximately equal to that of said hygroscopicsubstance, a substance selected from the group consisting of calciumcarbonate and bicarbonate and sodium carbonate and bicarbonate.
 5. Aliquid scintillator according to claim 1, in which alcohol is added tosaid scintillator in an amount between 100 and 400 c.c. per liter ofsaid toluene.
 6. A liquid scintillator according to claim 1, in whichsaid activator is 2,5-diphenyl-oxazole and said spectrum shifter is 1,4-bis2-(5-phenyloxazolyl)benzene.
 7. A method of improving the countingefficiency, in the presence of water, of a liquid scintillator having anactivator and a spectrum shifter dissolved in toluene, comprisingintroducing into said liquid scintillator a hygroscopic substance in anamount sufficient to substantially absorb said water that is present andbeing soluble in said toluene at least in said amount, said hygroscopicsubstance being selected from the group consisting of cellulose,carboxymethyl cellulose, cellulose phosphate, diethylaminoethylcellulose, benzyl diethylaminoethyl cellulose, epichlorohydrintriethanolamine cellulose, and mixtures thereof.
 8. The method accordingto claim 7, in which said amount of the hygroscopic substance issufficient to provide a saturated solution thereof in said toluene. 9.The method according to claim 7, in which said amount of the hygroscopicsubstance is in the range between substantially 1.0 and 10.0 gm. perliter of said toluene.
 10. The method according to claim 7, in whichthere is further added, in an amount approximately equal to that of saidhygroscopic substance, a substance selected from the group consisting ofcalcium carbonate and bicarbonate and sodium carbonate and bicarbonate.11. The method according to claim 7, further comprising adding to saidscintillator an amount of alcohol that is between 100 and 400 c.c.thereof per liter of said toluene.
 12. The method according to claim 7,in which said activator is 2,5-diphenyl-oxazole and said spectrumshifter is 1,4-bis 2-(5-phenyloxazolyl)-benzene.